The structured light 3-D shape measurement is a kind of technique available for whole-field measurement of object shape. It can adopt parallel optical axes structure or cross-optical axes structure. The structured light in question includes projected grating and projected moire. These two are often used as the same owing to indefinite difference other than the quantity of gratings between them.
Takade and other persons publish the “Fourier Transform Profilometry For The Automatic Measurement Of 3-D Object Shapes” on Applied Optics (vol.22, No.24, Dec. 15, 1983,P3977-3982), and Opton Company shows these two structures in the Moiré Report issued on its website. Drawing 1 is sketch of parallel optical axes structure, in which the ray from light source 1 is on grating 2, projective imaging lens 3 projects the grating line image onto object surface 5, 4 is virtual reference plane. CCD camera images object surface 5 with deformed grating fringes onto CCD target 7 by observer imaging lens 6. The key point of this structure is that projection device 8's optical axis 9 is coplanar with and parallel to CCD camera's optical axis 10, so the projected grating lines received by CCD camera are coplanar contours, which can facilitate to provide intuitional information about height, but the grating must be placed far away from the optical axis of projector so as to image the grating within the field range of observe camera. Parallel optical axes structure has following problems: (1) It is difficult to keep optical axes parallel; (2) the central parts of optical components of projector and camera are out of work, for the large aberration on their edges will bring about large measuring error.
Cross-optical axes structure is showed in Drawing 2. Its components are same to parallel optical axes structure. The ray from light source 1 is on grating 2, projective imaging lens 3 projects the grating line image on object surface 5, CCD camera images the object surface 5 with grating fringes on it onto CCD target 7 by observer imaging lens 6. The difference lies in that projection device's optical axis 9 crosses CCD camera's optical axis 10 at the point 0 on virtual reference plane 4 to form conjugate image of grating line. Because the image of grating on virtual reference plane 4 are not uniformly spaced fringes, unless pupil is at infinite distance, i.e., telemetric optical system. The projected grating lines received by CCD camera is non-coplanar contours. Cross-optical axes structure is easy to be produced and it utilizes effectively projection device and CCD camera's field of view to decrease measuring error.
American U.S. Pat. No. 5,175,601 declares a high-speed 3-D surface measurement, surface inspection and reverse CAD system. It uses cross-optical axes projected grating to measure 3-D object shape in a fixed field of view.
3-D shape phase measurement technique often adopts FFT and phase-shift methods to deal with data. In the case of FFT, the initial phase modulation on reference plane can be removed automatically, so FFT is applicable for both parallel optical axes structure and cross-optical axes structure. However, because the high-frequency components are filtered with FFT, the resolution of small shape changes at hole or edge is lower.
The phase-shift method of technique above, compared with FFT, has a higher depth resolution and horizontal resolution, and also can measure small shape changes such as hole and edge of surface. But phase-shift method is only applicable for parallel optical axes structure.
There are two puzzles in the application of phase-shift method on cross-optical axes 3-D shape measurement technique. (1) Like that in Scanning moiré method and automatic measurement of 3-D shapes by Masanori Idesawa on Applied Optics (vol. 16, No.8 August1977, pp2152-2162), the contours described by grating fringes on object surface form a function of fringe order, and the difference in height of contours is not equidistant, instead, it is also a function of fringe order. Therefore, in order to get absolute measurement of 3-D object shape, the absolute fringe order of grating lines must be measured accurately, that is to say, to determine the position of zero-order fringe. But at present we have not the equipment or method to determine the position of zero-order fringe. (2) As a whole-field-of-view measuring device, its optical system shall have a variable field of view. Under a certain field of view, the distances from projection optical system and viewing optical system to object (object distance) and to projected grating and observation plane (image distance) need be measured accurately. But all these can't be settled by technology of today. As a result, the existing cross optical axes measuring device takes a field of view with a certain operating distance as fixed field of view, and gives parameters under the said field of view and marks corresponding relationship between phase and height. Because the calibrated error will influence measurement accuracy, the high-precision whole-field measurement of 3-D object shape is impossible.